Journal of Engineering Science and Technology
Vol. 8, No. 5 (2013) 639 - 653
© School of Engineering, Taylor’s University
MICROBIAL CORROSION OF MILD
AND MEDIUM CARBON STEELS
J. E. O. OVRI*, S. I. OKEAHIALAM, O. O. ONYEMAOBI
Department of Materials and Metallurgical Engineering,
Federal University of Technology, Owerri, Nigeria
*Corresponding Author: [email protected]
Abstract
The role of bacteria in the corrosion of mild and medium carbon steels is
reported. The steels were exposed to anaerobic and aerobic, and fresh water
(control) environments. The corrosion rates were evaluated at intervals of seven
days for a period of 42 days using weight loss and electrochemical methods.
The corroded specimens were visually examined and majorities were found to
have undergone general corrosion in the three environments (aerobic,
anaerobic, and fresh water). The mild steel was found to corrode more than the
medium carbon steel in anaerobic environment-mild steel: 6.43×10-4 mpy and
-0.93 mV, due to limited available oxygen whilst it had -0.89 mV in aerobic and
-0.77 mV in the fresh water. The medium carbon steel had -5.30×10-4 mpy and
-0.91 mV in anaerobic: -0.84mV in aerobic and -0.74mV in freshwater.
Keywords: Bacteria, Corrosion, Carbon steel, Freshwater.
1. Introduction
Corrosion due to the activities of micro-organisms is referred to as microbial
corrosion. These organisms are involved in the corrosion process by the virtue of
their growth and metabolic actions and their presence often provide concentration
cells in the structure where they are present, whereby some areas in the structure
are anodic to the rest. These bacteria are therefore classified into two- those that
require oxygen in the metabolic and growth processes often referred to as aerobic
bacteria and those that carry out their activities in the absence of oxygen often
referred to as anaerobic [1]. These bacteria are active in stagnant water mainly at
bottom of tanks, the soil, freshwater, seawater and air.
The mere detection of sulphur reducing bacteria (SRB) does not constitute
a problem but what is important is the number of micro-organisms of the relevant
639
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J. E. O. Ovri et al.
Nomenclatures
A
Cfu/ml
CO2
CR
Cu/CuSO4
D
Dh
eFe
Fe(OH)
FS
H
HH2O
L
OHpH
SSO42T
W
WT.LOSS
Total surface area of coupons, cm2
Colony forming units per millimetre
Carbon-dioxide
Corrosion rate, mpy
Copper/ Copper sulphate reference electrode
Density, g/cm2
Diameter of hole on specimen, cm
Electron
Iron
Ferrous Oxide
Iron sulphide
Thickness of specimen, cm
Hydrogen ion
Water
Length, cm
Hydroxy ion
Measure of acidity and alkalinity of a solution
Sulphide ion
Sulphate ion
Exposure time, hours
Width of specimen, cm
Weight loss, g
Abbreviations
Ep
MCS
mpy
MS
SRB
Electrode Potential
Medium carbon steel
Mill per year
Mild steel
Sulphur reducing bacteria
types generated. It has been observed that under favourable conditions, bacteria
can multiply every 10- 60 minutes in their metabolic and growth process up to
2.8×1014 organisms which lead to the corrosion and biodeterioration of materials.
However, this growth and reproduction process is generally reduced after the first
bloom since conditions progressively become inhibitory; predators become
evident [1]. These organisms thrived in medium with pH values between 0 and
11, temperature between 0 and 80oC (30 and 180 F) and under the pressure of up
to 103 MPa (15 ksi). The activities of bacteria are affected by the availability of
nutrient e.g. phosphate, nitrogen, and presence of toxic substances (oxygen for
sulphate reducers, chlorine, biocides). SRB is the most important group of
bacteria associated with this form of corrosion and it is widely believed that all
microbial- induced failures are due to its activities. SRB are anaerobic and
heterotrophic, i.e., they obtain their energy from organic nutrients [1-11].
Their respiratory utilizes sulphate, which is abundant in fresh water, sea water
and soils and often results in the formation of hydrogen sulphide. This hydrogen
sulphide appears as H2S (in the dissolved or gaseous form).HS- ions, S2- ions or
metal sulphides, or a combination of these according to the prevailing conditions.
Journal of Engineering Science and Technology
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Microbial Corrosion of Mild and Medium Carbon Steels
641
The SRB in the presence of organic materials is oxidized to form the sulphide
[12-28]
4H2 + SO42- = S2- + 4H2O
(1)
The sulphide is highly corrosive to most engineering materials. The most
commonly encountered SRB is desulfovibrio. In the industrial process plant, a
wide range of bacteria such as pseudomonas and falvo bacterium exist. These can
secret large amount of organic material under both aerobic and oxygen free
(anaerobic) conditions. The petroleum industry has been plagued by the activities
of bacteria (SRB) because it handles large volumes of deaerated water from
underground reservoirs.
Aerobic bacteria grow due to the assimilation of CO2, by using the energy
derived from the oxidation of sulphur, sulphate, thiocynate, and trio and tetraathionate for their growth. These are chemolithotrophs or authotrophs, i.e., the
ability to grow on CO2 as opposed to chemo-organtrophs whose energy for
growth is from organic materials. The aerobes that cause this type of corrosion are
the bacillus of which thiobacillus and thio-oxidans are examples. They are
generally found in sulphur-bearing organic waste product, in soil and water with
low pH in the immediate vicinity [29-34].
The thiobacilli oxidizes the elemental sulphur to produce sulphric acid as
shown in the reaction below:
2S + 3O2 + 2H2O = 2H2SO4
(2)
The present study was undertaken to quantitatively determine the corrosion of
medium and high carbon steel in aerobic and anaerobic conditions.
2. Materials and Methods
2.1. Materials
The mild and medium carbon steels were obtained from Delta steel Company
Limited, Ovwian-Aladja, Delta State, Nigeria. They were in forms of rods of
diameter of 12 mm and were cut to coupons of nominal dimensions of 73×22×5
mm using a hacksaw blade. All cut edges were grinded to produce a good
surface, finishing with emery cloths of different grades. A hole of diameter
7.0 mm was drilled on the steel coupons to enhance their ease of suspension in
the different media.
The chemical compositions of the steels as supplied by the manufacturer are
given in Table 1.
Table 1. Chemical Composition of the Steels as Supplied by the Manufacturer.
Steel Grade
C
Si
Mn
Cu
P
S
Cr
Ni
Sn
Fe
MS (wt %)
0.18
0.23
0.80
0.20
0.03
0.04
0.05
0.55
0.02
Bal.
MCS (wt %)
0.40
0.25
0.52
0.05
0.03
0.02
-
-
-
Bal.
Key: MS = Mild Steel, MCS = Medium Carbon Steel.
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J. E. O. Ovri et al.
2.2. Preparation of the environments
Three environments were used in this research;
(i) Anaerobic sulphur reducing bacteria.
(ii) Aerobic sulphur reducing bacteria, and
(iii) Fresh water (control).
2.3. Preparation of bacterial environment
5g of sulphur was measured and buried in the soil for seven days. The sulphur
powder mixed with soil sample was collected after seven days. Then, 1g of the
soil was weighed and serially diluted. Nutrient agar medium was prepared by
mixing the compositions in Table 2 and was added to 1g of sulphur and allowed
to solidify. 0.1ml of the fifth serially diluted soil sample was pipette on
the nutrient agar plate. It was spread with a spreader and incubated
anaerobically. The bacterial of the genera desulphurvibrio was isolated from
anaerobic incubation. The freshwater was obtained from a borehole within the
university environment.
Table 2. Compositon of Nutrient Algae used for
the Preparation of the Bacterial Environment.
Compound
Composition (g)
NaCl
10
MgSO47H20
0.3
KCl
0.29
KH2PO4
0.83
Na2HPO4
1.25
NaNO3
0.42
Agar
12.5
2.4. Experimental Procedure
2.4.1. Corrosion rate measurement
The corrosion rate of the coupons was monitored by total immersion/weight loss
technique and electro-potential method. The coupons were cleaned and dried in
acetone before immersion. The initial weights of the coupons were taken with a
digital electronic weighing machine (contech) to the nearest 0.001g and were
noted. The samples were then totally exposed to the environments containing
aerobic and anaerobic sulphur reducing bacteria.
Suspension was done with the aid of a rubber thread which passes through the
hole in each coupon. The initial electro-potential of each of the coupons was taken
using the copper/copper sulphate (Cu/CuSO4) reference electrode connected to a
digital multi-meter of the model DT 9205, and the results were noted. A coupon
was removed from each media each week till the end of the experiment.
The coupons were removed from the environments at interval of seven days.
The oxide film formed on the surface of the coupons were examined visually,
brushed with a sponge in tap water and then dipped in acetone and then dried.
The final weight of the test specimen removed from the control and other
environments was taken and recorded using electronic weighing machine. The
coupons were then discarded. The procedure was repeated at intervals of seven
days for a total period of six weeks (42 days).
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Microbial Corrosion of Mild and Medium Carbon Steels
643
2.4.2. Visual examination
Samples were carefully examined visually during each monitoring day with the
aim of identifying:
(i) The type of substances or the nature of the oxide scales on the surface of
the specimens.
(ii) The type of corrosion occurring.
(iii) Any changes in the colour of the corrosion products.
2.4.3. Chemical test
Chemical test was carried out to indicate the type of corrosion. This was done
by gently scrapping the corroding surface and putting it in a test tube.
Concentrated hydrochloric acid (HCl) was then added to it. The mixture was
heated in a gas burner.
2.4.4. Bacteria concentration and pH of the environment
The bacteria concentrations in colony forming units per milliliter (cfu/ml) were
taken at an interval of seven days for a period of six weeks using colony counter.
Bacterial concentration increases with exposure time (days). The pH of the
environment was taken along each time potential readings were taken. The pH
readings were made using a pH meter (MAC Type).
3. Experimental Results
3.1. Weight loss
The corrosion behavior of mild and medium carbon steels in aerobic, anaerobic
environments and in distilled water which serves as a control was studied by
monitoring the electro-potential and weight loss of the specimens at intervals of
seven days.
The results of the weight losses of the steel coupons were converted to
corrosion rate in penetration per year and are given in Table 3 and plotted in Figs.
1 to 6.
3.2. Corrosion rate
The corrosion rates of steels using weight loss method in the various
environments were calculated using the formula [2]
CR = 534W/DAT
(3)
where, CR is the corrosion rate in mill per year (mpy), W is the weight loss
in grams (g), D is the density of steel (g/cm3), A is the total surface area (cm2),
and T is the exposure time in hours (hrs).
The results are given in column 2 of Table 3 for each week. Details of
calculation are shown in Appendix A.
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J. E. O. Ovri et al.
3.3. Potential measurement
The corrosion rate measurement was made using the electrical properties of a
corroding system. For a corroding system the electrical potential and
consequently the current and resistance are affected [36].
Using Ohm’s law
V = IR
(4)
where V is the potential in voltage, R is the resistance in ohms and I is the
current in Amperes.
The voltage decreases as the resistance increases due to the corrosion product
formed on the steel surface. Hence, the lower the potentials, the more the
corrosion is.
3.4. Chemical test results
A colourless gas with a rotten egg odour evolved with a black residue of iron II
chloride. The rotten egg odour of H2S indicated that SRB was responsible for the
corrosion of these steels.
2HCl + FeS = H2S + FeCl2
(5)
3.5. Visual examination
The steel samples exposed to the various environments were examined visually.
General (uniform) corrosion and pitting were observed for all the steel samples
exposed to aerobic, anaerobic and the control.
4. Results and Discussion
The corrosion rate of mild steel and medium carbon steels depends on several
corrosion parameters such as chemical composition, surface finish, and pH.
Anaerobic, aerobic and fresh water contain corrosion inducing ions such OH- and
SO42-. Therefore these environments are considered aggressive and toxic to most
engineering materials including steel.
Relating between laboratory test, chemical analysis, and visual examination,
the results of the investigation are discussed under the following headings: weight
loss of sample, and corrosion rates measurement.
4.1. Weight loss of sample
Figure 1 shows the weight loss of the steel samples as a function of the
environment and exposure time (days). Mild steel and medium carbon steel were
found to corrode in different environment containing SRB investigated, and this
was evidenced by the decrease in the original weight (weight loss).
Mild steel has the highest weight loss in all the environments investigated;
recorded (156 mg) in anaerobic, (130 mg) in aerobic environment and (82 mg) in
Journal of Engineering Science and Technology
October 2013, Vol. 8(5)
Microbial Corrosion of Mild and Medium Carbon Steels
645
the control. As could be seen in Table 4 and Fig. 1, the weight loss of mild steel
coupons in all the environments increased within the first 28th days. This
observation was attributed to the fact that the rate of a chemical reaction increases
with time [16, 35]. However, between the 35th and 42nd days of the investigation,
mild steel in all the environments showed a sharp decrease in weight loss with
time. This behaviour could be explained from the concept of passivation [2, 36].
Medium carbon steel has the least weight loss, recorded (136 mg) in
anaerobic, (98 mg) in aerobic environment and (76 mg) in the control. It was
generally observed (Table 5 and Fig. 1) that the weight loss of medium carbon
steel coupons in the entire environment increased with time in the first 28th
days. Also, between the 35th and 42nd day of the investigation, medium
carbon steel in all the environments showed sharp decrease in weight loss
with time. These observations are attributed to the same behaviour reported
for mild steel above.
The higher weight losses associated with anaerobic environment was partly
due to the high rate of permeability and diffusion of corrosion-inducing agents
such as OH- and SO42- ions through the protective barrier oxide film formed,
while in aerobic environment the rate of permeability and diffusion of these
corrosion-inducing ions is slow. The presence of SO42- ion which is reduced by
SRB to produce H2S, ferrous sulphide and other corrosion products can be
illustrated in the following equations:
Electrolytic dissolution of water
Anodic reaction
8H2O = 8OH- + 8H+
2+
4Fe = 4Fe + 8e
+
-
-
(6)
(7)
Cathodic Reaction
8H + 8e = 8H2
(8)
Cathodic Deplorization by Bacteria
SO42- + 8H2 = S2- + 4H2O
(9)
Corrosion Product
2+
2-
Fe + S = FeS
2+
-
Corrosion Product
3Fe + 6OH = 3Fe(OH)2
Overall Reaction becomes
4Fe + SO42- + 4H2O = FeS + 3Fe(OH)2 + 2(OH)-
(10)
(11)
(12)
The corrosion products such as FeS and 3Fe(OH)2 do not form protective
layers as discussed earlier, hence continuation in the corrosion process.
4.2. Corrosion rate measurement
(i) Weight loss method:
The corrosion rate was higher for mild steel, followed by the medium carbon
steel. It was generally observed (Fig. 2) that corrosion rate decreases with
exposure times in all the environments. This experimentally observed behaviour
was due to the formation of an impermeable protective oxide film on the surface
of the steel. In this state, the steel is said to be passive [36-38] and does not
corrode easily.
Mild steel corroded more than medium carbon steel in the first 28th days due
to high rate of permeability of corrosion inducing agents such as OH- and SO42-,
while after 28th days the aggressive ions in the various environments could not
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October 2013, Vol. 8(5)
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J. E. O. Ovri et al.
be absorbed by the protective oxide film, hence the decrease in the corrosion
rate with higher exposure times for all the environments studied. When steel is
exposed to a corrosive environment there is usually rapid corrosion, but this
decreases with time due to the formation of corrosion products which may be an
oxides, sulphates, carbonates or nitrides. If this product is not removed from the
surface of the steel the corrosion rate is reduced. In this study, the corrosion
inducing agents (OH-and SO42-) could not easily diffuse through the corrosion
products formed on the steel surfaces at exposure times greater than the 28th
days. The time taken by the corrosion inducing agents to diffuse through the
oxide (corrosion products) to the steel, i.e., the base metal contributed
significantly to the observed difference in the corrosion rates. These
observations are in qualitatively agreement with Ovri, [2, 23, 36] on steel
reinforcements in concrete and ferrite.
(ii) Electro-chemical method:
The plot of the electrode potentials with exposure times of the steel coupons in all
the environments investigated is given in Fig. 2. It was observed that the electrode
potential of the steel exposed to all the environments decreased with time during
the first 28th days of the investigation and later increased till the end of the study.
The electrode potential (voltage) decreased as resistance increased due to
corrosion product formed on the steel surface [36, 39]. Hence, the lower the
potential, the more the corrosion.
Mild steel in anaerobic environment recorded the highest decrease in
electrode potential in the first 28th days of the study. It had initial potential of
-0.47 mV)in the first day and -0.93 mV in the 28th days. This decrease in
electrode potential shows increase in corrosion rate and weight loss. However, a
sharp increase in electrode potential occurred from the 35th to the 42nd days
(-0.86 mV to -0.85 mV), showing a decrease in the corrosion rate. This
effect is similar to all results obtained for mild steel in aerobic and the
control environment.
Medium carbon steel in anaerobic environment was next after mild steel. It
had -0.66 mV in the first day and -0.91 m) in the 28th day which is an indication
of increase in corrosion rate. However, the electrode potential increased from
-0.83 mV to -0.82 mV in the 35th to 42nd days respectively, showing decrease in
corrosion rate and weight loss. This effect is similar to all results obtained for
medium carbon steels in aerobic environment and the control.
The observed increase in electrode potential after 28 days of the test was
attributed to the concept of passivity. This was caused by the formation of
protective oxide film on the surface of the steels. Due to chemical, physical and
electronic properties of these oxide films, metal dissolution is lowered resulting to
the decrease in corrosion rate. The mild steel in anaerobic environment exhibited
lowest electrode potential (highest negative potentials) indicating both high
weight losses and corrosion rate, followed by aerobic and control respectively.
The results are in agreement with those reported by Owate, et al. [16] for the
corrosion of mild and high carbon steels.
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October 2013, Vol. 8(5)
Microbial Corrosion of Mild and Medium Carbon Steels
647
(iii) The pH:
The pH decreased in the first 28th days indicating high corrosion rate and weight
loss. It could be seen in Fig. 6 that weight loss increased as the pH is decreased
(lowered) making the environment more acidic and consequently more corrosive.
However, between the 35th and 42nd days, the pH increased. This is due to the
presence of protective oxide film that passivate the steel surface. The passivity of
steel is reduced as the pH is increased [16].
Table 3. Weight Loss (g), Corrosion Rate (mpy)
and Potential (mV) in Various Environments.
0 day
Control
MCS
Control
MS
Anaerobic
MCS
Anaerobic
MS
Aerobic
MCS
Aerobic
MS
Weight Loss Method
21 days
28 days
Wt
Wt
CR
CR
Loss
Loss
(mpy)
(mpy)
(g)
(g)
7 days
Wt
CR
Loss
(mpy)
(g)
14 days
Wt
CR
Loss
(mpy)
(g)
0.00
0.030
2.94
0.051
2.50
0.076
2.35
0.076
0.00
0.042
4.15
0.069
3.41
0.076
2.60
0.082
0.00
0.00
0.054
05.30
0.088
4.31
0.125
4.08
0.136
0.00
0.00
0.065
6.43
0.098
4.84
0.140
4.61
0.152
3.76
0.150
2.95
0.140
2.31
0.00
0.00
0.49
5.00
0.076
3.72
0.089
2.91
0.098
2.40
0.083
1.76
0.085
1.40
0.00
0.00
0.053
5.24
0.084
4.15
0.105
3.46
0.130
3.21
0.120
2.37
0.116
1.91
Electrode Potential Method
14
21
28
35
days
days
days
days
EP
EP
EP
EP
(mV)
(mV)
(mV)
(mV)
42
days
EP
(mV)
Wt
Loss
(g)
CR
(mpy)
0.00
0.00
Control
MCS
Control
MS
Anaerobic
MCS
Anaerobic
MS
Aerobic
MCS
Aerobic
MS
35 days
Wt
CR
Loss
(mpy)
(g)
42 days
Wt
CR
Loss
(mpy)
(g)
1.86
0.076
1.49
0.074
2.03
0.080
1.58
0.078
1.29
3.33
0.132
2.59
0.128
2.09
0
days
EP
(mV)
7
days
EP
(mV)
-0.47
-0.65
-0.70
-0.74
-0.74
-0.72
-0.73
-0.55
-0.67
-0.73
-0.76
-0.77
-0.74
-0.74
-0.66
-0.77
-0.83
-0.90
-0.91
-0.83
-0.82
-0.70
-0.83
-0.87
-0.95
-0.93
-0.86
-0.85
-0.54
-0.70
0.77
-0.82
-0.84
-0.76
-0.75
-0.62
-0.75
-0.81
-0.88
-0.89
-0.79
-0.79
1.21
Key: Envt.- Environment, Wt. Loss - Weight Loss, CR- Corrosion Rate,
EP – Electropotential.
Table 4. Results for pH Measurement in Anaerobic,
Aerobic and the Control Environments.
Environment
Control
Anaerobic
Aerobic
pH 0-day
6.90
6.80
6.85
7-day
6.72
6.05
6.40
14-day
6.40
5.50
6.35
21-day
5.78
5.00
5.25
28-day
5.78
4.50
5.00
35-day
6.32
5.50
6.00
42-day
6.80
6.00
5.85
Table 5. Results of Bacterial Concentration of
Anaerobic and Aerobic Environments.
Bac.Conc
Anaerobic (cfu/ml)
Aerobic (cfu/ml)
0-day
300
200
7-day
350
250
14-day
400
300
21-day
450
350
Journal of Engineering Science and Technology
28-day
500
380
35-day
530
400
42-day
550
450
October 2013, Vol. 8(5)
648
J. E. O. Ovri et al.
160
0.160
140
0.140
120
0.120
Weight Loss (mg)
100
0.100
80
0.080
60
0.060
WT Loss Control MCS
WT Loss Control MS
WT Loss Anaerobic MCS
40
0.040
WT Loss Anaerobic MS
WT Loss Aerobic MCS
WT Loss Aerobic MS
20
0.020
0.0000
17
214
321
4 28
5 35
6 42
Exposures Time (Days)
Fig. 1. Variation of Weight Loss in (mg) with Exposure Time in (days)
for Steel Coupons Exposed to Aerobic, Anaerobic and the Control.
Fig. 2. Variation of Potential (-mV) with Exposure Time (days)
for Steel Coupons Exposed to Aerobic, Anaerobic and the Control.
Journal of Engineering Science and Technology
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Microbial Corrosion of Mild and Medium Carbon Steels
649
7
6
Corrosion Rate (mpy)
5
Corrosion Rate Control MCS
4
Corrosion Rate Control MS
Corrosion Rate Aerobic MCS
Corrosion Rate Aerobic MS
Corrosion Rate Anaerobic MCS
Corrosion Rate Anaerobic MS
3
2
1
0
7
1
14
2
3
21
4
28
5
35
42
6
7
Exposure Time (Days)
Fig. 3. Variation of Corrosion Rate in (mpy) with Exposure Time (days)
for Steel Coupons Exposed to Aerobic, Anaerobic and the Control.
0.16
160
140
0.14
120
0.12
100
Weight Loss (mg)
0.1
80
Weight Loss Anaerobic MS
0.08
Weight Loss (mg) Anaerobic MCS
60
0.06
40
0.04
20
0.02
0
0
300
1
350
2
400
3
4
450
5
500
550
6
Bacteria Concentration (cfu/ml)
Fig. 4. Variation of Weight Loss in (mg) with Bacteria Concentration
(cfu/ml) for Steel Coupons Exposed to Anaerobic Environment.
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650
J. E. O. Ovri et al.
7
Corrosion rate (mpy) Anaerobic MCS
Corrosion rate (mpy) Anaerobic MS
6
Bacterial Concentration
5
4
3
2
1
0
1
2
3
4
5
6
Corrotion Rate (mpy)
Fig. 5. Variation of Bacterial Concentration with Corrosion
Rate for Steel Coupons Exposed to Anaerobic Environment.
0.16
160
140
0.14
Weight Loss Anaerobic MS
Weight Loss Anaerobic MCS
120
0.12
Weight Loss (mg)
100
0.1
80
0.08
60
0.06
40
0.04
20
0.02
0
0
4.51
5.0
2
5.5
3
4 6.0
56.5
P.H.
Fig. 6. Variation of Weight Loss with pH for Steel
Coupons Exposed to Anaerobic Environment.
5. Conclusions
The following conclusion can be drawn from this investigation:
• Mild steel was observed to be more corrosive in the entire environments
(6.43×10-4 mpy). This is due to high rate of permeability of corrosion inducing
agents such as OH- and SO42-. This was followed by medium carbon steel
(5.30×10-4 mpy) with low rate of diffusion of the corrosion agents.
Journal of Engineering Science and Technology
October 2013, Vol. 8(5)
Microbial Corrosion of Mild and Medium Carbon Steels
651
• General (uniform) corrosion was observed for mild and medium carbon
steels in the entire environments investigated.
• The passivity of the steels is reduced as the pH is lowered.
• It has been shown that corrosion rate tends to decrease with increasing
carbon content.
• Based on these results, medium carbon steel is suggested for use in oil
industries when the environment contains SRB.
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Appendix A
Calculation of the Corrosion Rate
The total surface area, A, of the rectangular steel coupons used in the
investigation was calculated using the following relationship.
A = 2(LW + LH +WH] – 2 πDh2/4)
where
A is the total surface area of coupons, cm2
L is the length =
7.30 cm
W is the width =
2.20 cm
H is the thickness =
0.5 cm
Dh is the diameter of holes =
0.7 cm
A = 2(7.3 × 2.2 + 7.3 × 0.5 + 2.2 × 0.5) – (3.142 × (0.7)2/4)
= 2(16.06 + 3.65 + 1.10) – 2(0.385) = 40.85 cm2
The corrosion rate (CR) is calculated using the relationship.
CR = 534W/DAT
where W is the weight loss (g), D is the density of metal (g/cm3), A is the total
surface area of coupons (cm2), and T is the exposure time (hrs).
CR (Control 7 days MCS) = 534 × 0.030/(7.87 × 40.85 × 168)
= 16.02/54010.24
= 2.94×10-4 mpy
CR (Control 7 days MS) = 534 × 0.042/(7.87 × 40.85 × 168)
= 4.15×10-4 mpy
Journal of Engineering Science and Technology
October 2013, Vol. 8(5)